In order to meet the growing demands for miniaturized devices and electronic equipment and gadgets containing the devices, the semiconductor industry is moving fast from micro-electronics to nano-electronics. In order to accommodate this trend, process design must follow suite and become yet more sophisticated and cost-effective.
A specialized sector of the semiconductor industry is the printed circuit board (PCB) sector.
Forming a printed circuit board (PCB) involves many process steps. One of the final steps is coating of the PCB with solder mask to protect all features, apart from the conducting elements or pads from environment attack. Solder masks may be glossy, semi mat or mat. The mat effect is provided by incorporation of inorganic particles that cause the surface to be rough.
One essential process step in this industry is printing on the device to label it. This normally involves printing onto a solder mask. The printing may be achieved by indirect methods (through the mediation of a tool or mask or screen) or direct (digital printing—ink jet for example).
In the case of direct printing, ink jet inks are used to print onto the printed circuit board (PCB). An application that has already been commercialized is the digital printing of marking ink of a legend on the solder mask that covers the outer layers of a PCB. The ink is usually white and is cured by UV, by heat or by both.
In the case of printing onto a solder mask, high resolution is obtained only on glossy solder masks. When solder mask is semi mat or mat, the ink resolution is poor, due to propagation of ink before drying and/or curing.
Ideally, print quality should be independent of mask/substrate properties, though, in practice, the art does not teach how to realize this ideal.
Ink from an ink jet is characterized by low viscosity, usually less than 15 cp at the jetting temperature, and low surface tension, usually between 28 to 32 dynes/cm.
Due to these properties, the ink propagates easily on surfaces with a high surface tension, usually above 50 dynes/cm, and/or of a high surface area. Examples of such surfaces are mat and semi-mat solder resists of printed circuit boards, where the surface is characterized by high content of porosity and metal oxides particles.
On glossy surfaces having surface tension in the range of 28-50 dynes/cm, print quality is excellent. In the other hand, on glossy surfaces of a low surface tension, the ink tends to de-wet the surface.
For both too high and too low surface tension solder masks, the jet printed print tends to be of a poor quality.
For high surface tension substrates or masks, thin lines tend to broaden, lines spread into areas or domains, line sharpness is poor, ink thickness is reduced and characters consequentially become illegible.
For low surface energy substrates or masks, lines form droplets, pin-holes and “eyes” on the printed surfaces, and the characters also become illegible.
Some publications in the art include:
U.S. Pat. No. 6,754,551 to Zohar, et al., which describes a jet print apparatus and method for printed circuit board manufacturing. A jet dispensing print system for dispensing a liquid or viscous, jettable substance as a pattern onto the surface of a PCB in an industrial manufacturing PCB production line is described.
The system includes a printing system having a printing bridge system, which includes a static and rigid printing bridge to accommodate in a precise manner several jet print heads, each being fitted with a multitude of jet nozzles.
The jet print heads are utilized to achieve multiple redundancy, whereby part of the total amount of available nozzles are utilized as back-up nozzles. A printing table positioned underneath the static and rigid printing bridge, accommodating a vacuum table with area-addressable suction force, whilst the pattern is dispensed in a jetting manner onto the PCB.
A motorized system, for moving the printing table simultaneously in two perpendicular directions is disclosed, as well as an ink-supply system for supplying the several jet print heads with the liquid or viscous substance.
A control system acts in response to the pattern and PCB dimensional distortions for controlling the system in order to achieve a desired registration precision level for the dispensing of liquid or viscous, jettable substance as the pattern onto the surface of the PCB, and includes an user interface for providing a status report of the printing system.
The jet dispensing PCB print system is utilized for inter alia, legend print, solder mask, etch resist mask, plating resist, temporary (peelable) mask, adhesives, CSP and bare die encapsulation.
U.S. Pat. No. 7,166,156 to Parrinello, et al., discloses an ink recordable substrate coating composition having a pH less than 7. There is described a water resistant coating composition for ink jet recordable substrates having a pH of less than 7, which includes: (a) an aqueous polyurethane dispersion; and (b) an aqueous solution of a nitrogen containing polymeric dye fixative compound. When applied to a suitable substrate, the coating composition allows for the recording of sharp, waterfast images. A coated ink recordable substrate is also disclosed, which includes a substrate having at least one side and at least one side of the substrate has a coating layer derived from the above described coating composition.
There is thus a long felt need to provide a process that enables high digital printing quality on semi mat and mat solder masks.